Group presence … Tino Sehgal outside Tate Modern with participants in These Associations. Photograph: Johnny Green

He doesn't allow his work to be photographed or filmed, it can't be bought or sold in any conventional sense, and once it's over it's gone for ever. Yet despite not making shiny baubles designed to be hoovered up by oligarchs, this year 36-year-old British/German artist Tino Sehgal moved to the front rank of the art world with two major installations, one in Tate Modern's Turbine Hall in London; the other at Documenta 13, the massive art festival in Kassel, Germany which takes place every five years. Both pieces had a large cast, which used dance, performance, audience interaction and the lights going on and off to create an experience that was uplifting, enigmatic and endlessly absorbing.

Storyteller … Tino Sehgal. Photograph: Tate Modern

Documenta opened in June. Sehgal's piece was titled This Variation and took place in an almost completely dark room at the back of Huguenot House, a site tucked in a street away from the main galleries. Walking gingerly through the gloom, you suddenly realised you were among a number of people who had started talking to each other – intriguing bits of dialogue about the economy and then relationships. After a while they started beatboxing, which grew into a 3D rendition of – on the morning I was there – LCD Soundsystem's Get Innocuous, followed by the Beach Boys' Good Vibrations. Suddenly the lights came on and the performers who were dancing with and around you were revealed, before the switch was flicked again and the atmosphere subsided. The experience – somewhere between a club, a sleepover and a seminar – was so intimate and euphoric I could have stayed there all day. I couldn't stop talking about it afterwards.

Could Sehgal pull off something as effective in the enormous Turbine Hall a month later? At first, the intimacy seemed to have been diluted – thanks to the gift shop, the Turbine Hall never really gets dark even with the lights off. Yet These Associations worked an insidious magic. First the performers, who initially appeared to be other gallery-goers, stood in a circle around you – part challenge, part protection. Then they ran up and down the Tate's ramp, making patterns through the hall, threading between confused toddlers. Then, one of them would come over and start talking to you about the time a mate let them down and the friendship never really recovered, or about a date they had with a woman they'd been after for ages. They didn't seem to need revelations in return, though no doubt they would have been welcomed, but just a friendly ear.

Some people were embarrassed by it, but to me it was touching to think that if you were lonely, you could have come down to Tate Modern for some human interaction. Call me gullible, but it seemed genuine. Working at the intersection of dance, conceptual art and storytelling, Sehgal created something that seemed unprecedented – a piece that you transformed by participating in, which was kaleidoscopically changing, seemed global in reach and scope, and which was infinitely generous to its audience.